The invention relates to a method and apparatus for calculating the actuation control of the hydraulic sub-system of a braking system in order to reduce noises associated with the hydraulics.
The hydraulics of a brake system consists of a number of pumps and valves assigned to each wheel of an automobile, all of which are controlled by a brake circuit. Proper actuation of the pumps and valves controls braking strength and degree for stopping of the automobile. A priming pump and a return pump generate a braking pressure, controlled by the brake circuit through a return valve and a pilot valve. The pressure is translated to individual wheel brakes by means of a discharge valve and an admission valve.
Control of these hydraulics by the brake circuit is through electronic signaling on the basis of assigned and/or calculated values. Assignment and/or calculation of these values is dependent upon such things as the inherent pump and valve characteristics. Thus, even while the brake circuit controls the valves and pumps, the condition of the brake circuit itself can be determined as a function of the valves (i.e., whether opened or closed, and to what degree) and as a function of the pumps (i.e., whether the pump is pressurized or not).
As a function of the signaling by the brake circuit, actuation of the valves and pumps results in vibration and translation noises. For instance, a conventional drive control of a solenoid valve in an FDR or ABS brake system specifics a maximum current (Vbatt/Rsolenoid)or minimum current (0.07) signal, switched over in intervals of 1 millisecond. The associated pressure gradients in operation of the valves and pumps generate substantial noises during slip control. The equilibrium of pressure, spring and solenoid forces results in a partially open (or partially closed) solenoid valve (i.e., it is not fully open as it is for a minimum current signal nor fully closed as it is for a maximum current signal). Such partial opening/closing reduces the pressure gradients such that the hydraulic noises of the brake system are reduced. However, the drive control calculations must therefore be made relatively precisely.
Accordingly, a method and apparatus for a calculation of an actuation control of a hydraulic-subsystem in a braking system in order to further reduce noise is presented. In an FDR/ESP hydraulic subsystem, calculation of a valve drive-time required for a desired pressure build-up is determined as a function of the pre-compression and estimated pump pressure from the known behavior of the characteristic curve for pump pressure versus the displaced brake fluid volume. This indicates how long the valves responsible for the brake cylinder must be opened. Of particular interest here is only the positive valve drive time calculated for the inlet valve with which the pressure build-up is controlled.
To reduce the noise generation within the slip control, the inlet valve should not be opened fully, rather it is only opened to a definable percentage. For this purpose, a pulse-width modulated (PWM) voltage is output via a power output stage, which is used to configure a defined current signal level. This current signal level corresponds to the force equilibrium across the valve body. Furthermore, to achieve this equilibrium state with the smallest possible delay the PWM drive is switched over to driving a smaller PWM value for a defined time; this is done to reduce the solenoid valve current to the desired level as quickly and precisely as possible.